Free-living eukaryotic microbes may reduce animal diseases. We evaluated the dynamics by which micrograzers (primarily protozoa) apply top-down control on the chytrid Batrachochytrium dendrobatidis (Bd) a devastating, panzootic pathogen of amphibians. Although micrograzers consumed zoospores (∼3 μm), the dispersal stage of chytrids, not all species grew monoxenically on zoospores. However, the ubiquitous ciliate Tetrahymena pyriformis, which likely co-occurs with Bd, grew at near its maximum rate (r = 1.7 d–1). A functional response (ingestion vs. prey abundance) for T. pyriformis, measured using spore-surrogates (microspheres) revealed maximum ingestion (Imax) of 1.63 × 103 zoospores d–1, with a half saturation constant (k) of 5.75 × 103 zoospores ml–1. Using these growth and grazing data we developed and assessed a population model that incorporated chytrid-host and micrograzer dynamics. Simulations using our data and realistic parameters obtained from the literature suggested that micrograzers could control Bd and potentially prevent chytridiomycosis (defined as 104 sporangia host–1). However, simulated inferior micrograzers (0.7 × Imax and 1.5 × k) did not prevent chytridiomycosis, although they ultimately reduced pathogen abundance to below levels resulting in disease. These findings indicate how micrograzer responses can be applied when modeling disease dynamics for Bd and other zoosporic fungi.

Metazoan disease control may require an understanding and inclusion of microbial ecology.We evaluated the ability of micrograzers (primarily protozoa) to control chytridiomycosis, a disease caused by the chytrid Batrachochytrium dendrobatidis, a devastating panzootic pathogen of amphibians. Although micrograzers consumed zoospores (~3 µm), the dispersal stage of chytrids, not all species grew monoxenically on zoospores; but the ubiquitous ciliate Tetrahymena pyrifomis, exhibited a growth rate of 1.7 d -1 and approached its maximum rate of growth. A functional response (ingestion vs. prey abundance) of T. pyrifomis measured on spore-surrogates (microspheres) revealed maximum ingestion (Imax) of 1.63 x 10 3 spores d -1 , with a half saturation constant (k) of 5.75 x 10 3 spores ml -1 . We then developed and assessed a population model that incorporated chytrid-host and micrograzer dynamics over 100 days.Simulations using T. pyrifomis data and realistic parameters obtained from the literature, suggested that micrograzers can control B. dendrobatidis and prevent chytridiomycosis (defined as 10 4 sporangia per host). However, inferior micrograzers (0.7 x Imax and 1.5 x k) were unable to prevent chytridiomycosis, although they did reduce pathogen abundance to negligible levels. These findings strongly argue that micrograzers should be considered when evaluating disease dynamics for B. dendrobatidis and other zoosporic fungi.